This disclosure relates to electroacoustic transducing with a bridged phase plug.
A compression driver is a type of electroacoustic transducer in which air is compressed in a compression cavity between a moving diaphragm and a fixed phase plug. Passages in the phase plug, referred to as slots, conduct air from the compression cavity to a listening environment, generally through a throat and a horn. The horn provides impedance matching between the air in the throat and air in the free space of the listening environment and controls the directivity of the radiated sound.
Several terms are defined with reference to FIGS. 1 and 2. For reference, directions such as “top” and “bottom” or “above” and “below” refer to the drawing itself with the top and bottom margins of the drawing sheet defining up and down. As installed, a phase plug could face in any direction. In a compression driver, the primary moving element is referred to as the dome 10. In some examples, the dome is a simple spherical section. In some examples, the dome has a complex curvature. The ends of the dome are formed into or joined to a cylindrical section called the skirt 12. The skirt is joined to a voice coil former or bobbin 14 and a surround 16, which is in turn fixed to the external structure 18. In some examples, the surround is formed from an extension of the dome, not a separate part. A voice coil 20 is wound around the bobbin and reacts to a magnet 22 and pole piece 24 to move the bobbin and dome when a current or voltage is applied to the voice coil. Above the dome is a rear cavity 26 bounded by a rear cavity wall 28. Below the dome is a front or compression cavity 30 bounded by a dome-interface surface 32 of a phase plug 34. Movement of the dome compresses air in the compression cavity. In the example of FIGS. 1 and 2, the dome, skirt, bobbin, surround, external structure, voice coil, magnet, and pole piece are shown abstractly and are not meant to represent any particular design or technology.
In a typical phase plug, exemplified in FIGS. 1 and 2, one or more slots 36a, 36b, 36c begin at the dome-interface surface of the phase plug and join at the throat 38, communicating the pressurized air from the compression cavity 30 to the throat 38. The throat is defined as beginning at the point where the multiple slots are completely joined in a single passage. While we refer to these passages as slots, due to their appearance in a two-dimensional section (e.g., FIG. 1), they are actually cone-shaped voids in the three-dimensional phase plug, bounded on top and bottom by cones of slightly different radius (if the slots taper in width, as they do in this example) and/or vertical position. In FIG. 2, each of 36a, 36b, and 36c is seen twice. Given the shape of the slots, the phase plug 34 is composed of several concentric cone-shaped solids 34a-34c and an outer cylindrical solid 34d, all joined and held in relative position by supports (not shown) within the slots. The slots 36a-36c couple the compression cavity 30 to the throat 38, which in turn couples to a horn (see FIG. 7).